The Digestive System for Middle School Science

Apr 8

6 min read

The human body is a huge topic! The digestive system is just one of the body systems that make the human body work. Our digestive system turns the food we eat into energy we can use to think, move, and live. It also gets rid of waste

You can check out the pages of my digestive system unit from Teachers Pay Teachers below. You can also continue on to read more about the digestive system.

The Digestive System

The digestive system is a vital part of the human body that allows us to turn the food we eat into the energy and nutrients we need to live and grow. Every time you eat a meal or drink a glass of water, your digestive system goes to work, breaking down food into smaller parts so your body can absorb what it needs and get rid of what it doesn’t. This complex system includes several organs that each play an important role in digestion, from the mouth all the way to the intestines. Without the digestive system, our bodies wouldn’t be able to get the fuel they need to function properly.

Functions of the Digestive System

The digestive system has three main functions:

Digestion – This is the process of breaking down food into smaller parts. It starts in the mouth and continues through several organs.
Absorption – Once food is broken down, the nutrients are absorbed into the bloodstream and carried to cells around the body.
Elimination – Any part of the food that the body can’t use becomes waste, which the system removes from the body.

These steps happen automatically, and each organ has a specific role to play along the way.

The Mouth

The mouth is the first stop in the digestive system and plays a key role in beginning the process of breaking down food. Several parts of the mouth work together to make this happen. The teeth are responsible for chewing food into smaller pieces, making it easier to swallow and digest. The tongue helps move the food around while chewing and then pushes it to the back of the mouth to be swallowed. The salivary glands produce saliva, which contains enzymes like amylase that begin breaking down carbohydrates even before the food leaves the mouth. Saliva also moistens food, forming it into a soft lump called a bolus, which is easier to swallow. Together, these parts of the mouth help start the digestion process by physically and chemically breaking down food and preparing it for its journey through the rest of the digestive system.

The Esophagus

The esophagus is a muscular tube that connects the mouth to the stomach and is responsible for moving food after it has been swallowed. Although it doesn’t digest food itself, it plays a very important role in getting food to where digestion continues. The walls of the esophagus are lined with smooth muscle, which is an involuntary type of muscle. This muscle pushes food down toward the stomach using a wave-like motion called peristalsis. These rhythmic contractions happen automatically, without you needing to think about them. At the end of the esophagus, there is a ring of muscle called the lower esophageal sphincter that opens to let food into the stomach and then closes to keep stomach acid from coming back up. Even though the esophagus is just a passageway, its smooth muscle and peristaltic movements are essential for helping food travel safely and efficiently through the digestive system.

The Stomach

The stomach is a strong, muscular organ that continues the digestive process by mixing and breaking down food into a thick liquid called chyme. It is located on the left side of the upper abdomen and can stretch to hold a large amount of food. The walls of the stomach are made of smooth muscle, which contracts in a churning motion to mix food with digestive juices. These juices include hydrochloric acid and enzymes like pepsin, which help break down proteins. Because the acid in the stomach is so strong, the inside is lined with a special coating of mucus to protect it from being damaged. Even with this protection, the stomach lining wears down quickly and needs to be replaced often—about every 3 to 4 days—to stay healthy and prevent ulcers. At the top of the stomach, the lower esophageal sphincter controls the entry of food from the esophagus, while at the bottom, the pyloric sphincter slowly releases the partially digested food into the small intestine. Although most nutrient absorption happens later in the digestive system, the stomach plays a key role in both mechanical and chemical digestion, preparing food for further breakdown and absorption.

The Small Intestine

The small intestine is the longest part of the digestive system and the main site where nutrients from food are absorbed into the body. It’s a narrow, coiled tube that can be up to 22 feet long in adults, and it's divided into three parts: the duodenum, jejunum, and ileum. Digestion continues in the duodenum, where important helper organs release substances that assist in breaking down food. The pancreas sends enzymes into the small intestine that help digest proteins, fats, and carbohydrates. It also releases bicarbonate, which neutralizes the strong stomach acid in the food coming from the stomach. The liver produces bile, a substance that helps digest fats, and the gallbladder stores and releases the bile into the small intestine when it’s needed.

The walls of the small intestine are lined with millions of tiny finger-like structures called villi, and even smaller structures called microvilli. These structures greatly increase the surface area for absorption, allowing nutrients to pass into the bloodstream and be delivered to cells throughout the body. Muscles in the small intestine continue to move food along using peristalsis, the same wave-like motion used in the esophagus. By the time food leaves the small intestine, most of the nutrients have been absorbed, and what remains moves into the large intestine to begin the final stages of digestion.

The Large Intestine

The large intestine, also called the colon, is the final part of the digestive system. While it doesn't digest food like the stomach or small intestine, it plays an important role in absorbing water, storing waste, and helping the body maintain a healthy balance of fluids and vitamins. By the time food reaches the large intestine, most of the nutrients have already been absorbed. What remains is mostly water, fiber, and waste that the body doesn’t need.

The large intestine is wider but shorter than the small intestine, measuring about 5 to 6 feet long. It uses smooth muscle to push waste along using a wave-like motion called peristalsis. As material moves through the colon, water and minerals are absorbed, and helpful bacteria break down some of the remaining substances. These bacteria also produce vitamins like vitamin K, which the body uses to help blood clot.

The large intestine is made up of several sections, each with its own job:

Cecum: This is the first part of the large intestine and connects to the small intestine through the ileocecal valve. It starts absorbing fluids and salts. The appendix is attached to the cecum, and while it isn’t essential for digestion, it may help support healthy bacteria.
Ascending Colon: This part moves waste upward on the right side of the abdomen. It continues absorbing water, salt, and any remaining vitamins from the waste material.
Transverse Colon: The waste moves across the upper part of the abdomen. Here, bacteria break down some undigested material in a process called fermentation, which can produce gas. More water and nutrients are absorbed in this section.
Descending Colon: Located on the left side of the abdomen, the descending colon stores the now more solid waste as it gets ready to leave the body.
Sigmoid Colon: This S-shaped section connects the descending colon to the rectum. It holds solid waste until it's time to go to the bathroom.
Rectum and Anus: The rectum stores the final waste material (called feces) until it’s time to be eliminated. When the body is ready, the anus, a ring of muscle at the end of the digestive tract, relaxes to allow the waste to exit.

Together, these parts of the large intestine help the body recover valuable water, form solid waste, and get rid of what it doesn’t need—all essential for keeping the body healthy and balanced.

How the Digestive System Interacts with Other Systems

The digestive system does not work alone—it depends on help from other systems in the body.

The circulatory system works closely with the digestive system by transporting absorbed nutrients from the small intestine to the cells that need them. Without the blood vessels carrying these nutrients, the body couldn’t use the food we eat.
The muscular system plays an important role in moving food through the digestive tract. The walls of the esophagus, stomach, and intestines are lined with muscles that contract and relax to push food along.
The nervous system helps control digestion. It tells your body when you’re hungry or full and helps control the speed at which your stomach and intestines work.
The excretory system helps remove the solid waste left over after digestion. The large intestine and rectum are key parts of this waste removal process.

Flashcards on the Digestive System

Flashcards are the best way to practice remembering new information. You can get digital and printable flashcards at Teachers Pay Teachers, or you can use the flashcards here for free!